I made a documentary about prospective cohort studies in epidemiology, they’re the tool we use to find out if one thing is associated with another, where trials are impossible. It’s really good. Instead of reading about it, listen here:

(Declaration: me and Mrs Bad Science both work on these kinds of datasets).

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If you like what I do, and you want me to do more, you can: buy my books Bad Science and Bad Pharma, give them to your friends, put them on your reading list, employ me to do a talk, or tweet this article to your friends. Thanks!
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